When you first launch Cambly chat in Google Chrome, the browser will ask to use your camera as seen below. Click 'Allow' and you should now see your video on the screen. If you do not, you may need to configure the settings for your device.
If you have previously logged in and did not allow the camera, you may need to enable it from the top right-hand corner of Google Chrome's address bar as seen below. Once you click the camera you will get the option to enable it by selecting the 'Always allow...' option.
You can also use the camera icon to change cameras (if you have multiple devices on your computer) or select a different microphone, if needed.